I think the trouble is that traditionally userspace builds didn't use the
actual kernel headers. In Fedora historically there was a glibc-kernheads
which had a set of header files not tracking any particular kernel release.
Looking at the way bridge-utils in FC5 worked, they patched in a custom
version of sockios.h to get access to the bridge constants. Perhaps we'll
have to do similar in libvirt or find a different way to call the bridge
stuff.